Metrology apparatus, lithographic system, and method of measuring a structure

ABSTRACT

A metrology apparatus is disclosed that has an optical system to focus radiation onto a structure and directs redirected radiation from the structure to a detection system. The optical system applies a plurality of different offsets of an optical characteristic to radiation before and/or after redirected by the structure, such that a corresponding plurality of different offsets are provided to redirected radiation derived from a first point of a pupil plane field distribution relative to redirected radiation derived from a second point of the pupil plane field distribution. The detection system detects a corresponding plurality of radiation intensities resulting from interference between the redirected radiation derived from the first point of the pupil plane field distribution and the redirected radiation derived from the second point of the pupil plane field distribution. Each radiation intensity corresponds to a different one of the plurality of different offsets.

This application claims the benefit of priority of European patentapplication no. 17196670, filed on Oct. 16, 2017. The content of theforegoing application is incorporated herein in its entirety bereference.

FIELD

The present description relates to a metrology apparatus for measuring astructure formed on a substrate by a lithographic process, relates to alithographic system, and relates to a method of measuring a structureformed on a substrate by a lithographic process.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofdevices such as integrated circuits (ICs). In that instance, apatterning device, which is alternatively referred to as a mask or areticle, may be used to generate a pattern (e.g., a circuit pattern) tobe formed on an individual layer of the device. This pattern can betransferred onto a target portion (e.g., including part of, one, orseveral dies) on a substrate (e.g., a silicon wafer). Transfer of thepattern is typically via imaging onto a layer of radiation-sensitivematerial (resist) provided on the substrate. In general, a singlesubstrate will contain a network of adjacent target portions that aresuccessively patterned.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, ameasure of the accuracy of alignment of two layers or two structures ina same layer, in a device. Overlay may be described in terms of thedegree of misalignment between the two layers, for example reference toa measured overlay of 1 nm may describe a situation where two layers aremisaligned by 1 nm.

Recently, various forms of scatterometers have been developed for use inthe lithographic field. These devices direct a beam of radiation onto atarget and measure values of one or more properties of the scatteredradiation—e.g., intensity at a single angle of reflection as a functionof wavelength; intensity at one or more wavelengths as a function ofreflected angle; or polarization as a function of reflected angle—toobtain a “spectrum” from which a property of interest of the target canbe determined. Determination of a value of the property of interest maybe performed by various techniques: e.g., reconstruction of the targetby iterative approaches such as rigorous coupled wave analysis or finiteelement methods; library searches; and principal component analysis.

In a known metrology technique, the −1^(st) and the +1^(st) diffractionorder intensity values are obtained from a target. The intensityasymmetry, a comparison of these diffraction order intensity values,provides a measurement of target asymmetry; that is, asymmetry in thetarget. This asymmetry in the target can be used as an indicator ofoverlay (e.g., undesired misalignment of two layers).

SUMMARY

Measurement of overlay (or other asymmetries in target structures) usingthe above metrology technique can be difficult where the structuresconcerned are at the resolution of device features to be manufactured.This is because high resolution features cause correspondingly highangles of diffraction, which are difficult to capture, or diffractionorders become evanescent (non-propagating). For structures defined bylayers that are very close to each other, such as may be the case afteretching has been carried out, it may still be possible to obtain someinformation about asymmetry from zeroth order scattering. However, itmay be difficult to obtain adequate sensitivity in such measurements,particularly, in the context of overlay between layers, where layerseparation is not very small.

It is desirable, for example, to improve measurement of target asymmetryor other parameters of interest, particularly for high resolutiontargets.

According to an aspect, there is provided a metrology apparatus formeasuring a structure formed on a substrate to determine a parameter ofinterest, the metrology apparatus comprising: an optical systemconfigured to focus radiation onto the structure and direct reflectedradiation from the structure to a detection system, wherein: the opticalsystem is configured to apply a plurality of different offsets of anoptical characteristic to radiation before and/or after reflection fromthe structure, such that a corresponding plurality of different offsetsare provided to reflected radiation derived from a first point of apupil plane field distribution relative to reflected radiation derivedfrom a second point of the pupil plane field distribution; and thedetection system is configured to detect a corresponding plurality ofradiation intensities resulting from interference between the reflectedradiation derived from the first point of the pupil plane fielddistribution and the reflected radiation derived from the second pointof the pupil plane field distribution, wherein each radiation intensitycorresponds to a different one of the plurality of different offsets.

According to an aspect, there is provided a method of measuring astructure formed on a substrate to determine a parameter of interest,the method comprising: focusing radiation onto the structure and using adetection system to detect reflected radiation from the structure,wherein: a plurality of different offsets of an optical characteristicare applied to radiation before and/or after reflection from thestructure, such that a corresponding plurality of different offsets areprovided to reflected radiation derived from a first point of a pupilplane field distribution relative to reflected radiation derived from asecond point of the pupil plane field distribution; and the detectionsystem detects a corresponding plurality of radiation intensitiesresulting from interference between the reflected radiation derived fromthe first point of the pupil plane field distribution and the reflectedradiation derived from the second point of the pupil plane fielddistribution, wherein each radiation intensity corresponds to adifferent one of the plurality of different offsets.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus;

FIG. 2 depicts a lithographic cell or cluster;

FIG. 3a shows a schematic diagram of a metrology apparatus for use inmeasuring targets using a first pair of illumination apertures;

FIG. 3b shows a schematic depiction of a detail of a diffractionspectrum of a target grating for a given direction of illumination;

FIG. 3c shows a schematic depiction of a known form of multiple gratingtarget and an outline of a measurement spot on a substrate;

FIG. 3d shows a schematic depiction of an image of the target of FIG. 3cobtained in the metrology apparatus of FIG. 3 a;

FIG. 4 schematically depicts optical elements of a metrology apparatusthat provides an input radiation beam to an optical unit comprising abeam splitter;

FIG. 5 schematically depicts the optical unit configured to receive theinput radiation beam from the arrangement of FIG. 4 and an opticalsystem configured to direct first and second radiation beams onto asubstrate and direct redirected first and second radiation beams fromthe substrate onto a detector;

FIG. 6 schematically depicts operation of an optical unit of thearrangement of FIG. 5 in further detail, showing pupil plane fielddistributions of radiation beams propagating to and from the beamsplitter;

FIG. 7 schematically depicts operation of a further optical unit basedon the optical unit of FIG. 6 with an additional flip being performed inthe first branch;

FIG. 8 schematically depicts a further optical unit in which radiationpasses through first and second beam splitters before and afterredirection by the target structure;

FIG. 9 schematically depicts an optical arrangement in which radiationpasses through first and second beam splitters only after redirection bythe target structure;

FIG. 10 is a graph depicting a typical variation of signal intensity asa function of an applied phase offset for targets with differentoverlay;

FIG. 11 is a graph depicting example measurements of signal intensityfor a plurality of discrete applied phase offsets;

FIG. 12 is a graph depicting example measurements of signal intensityincluding at least one negative applied phase offset and at least onepositive applied phase offset, and a best fit line;

FIG. 13 schematically depicts an optical unit implementing common pathinterferometry and comprising a polarization-dependent optical elementin the common path loop to apply amplitude and/or phase offsets;

FIG. 14 schematically depicts an optical unit implementing common pathinterferometry and comprising a variable retarder and λ/2 plate afterthe common path loop to apply amplitude and/or phase offsets;

FIG. 15 schematically depicts a detection system comprising aself-referencing interferometer configured to receive radiation afterredirection by a target structure and comprising a variable retarder andλ/2 plate to apply amplitude and/or phase offsets;

FIG. 16 schematically depicts a detection system configured to applymultiple different amplitude and/or phase offsets to radiation that hasbeen subjected to common path interferometry; and

FIG. 17 schematically depicts a detection system configured to applymultiple different amplitude and/or phase offsets to radiation that hasbeen subject to interference by a self-referencing interferometer.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment,” “an embodiment,” “an example embodiment,” etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a supportstructure (e.g., a mask table) MT constructed to support a patterningdevice (e.g., a mask) MA and connected to a first positioner PMconfigured to accurately position the patterning device in accordancewith certain parameters, a substrate table (e.g., a wafer table) WTconstructed to hold a substrate (e.g., a resist coated wafer) W andconnected to a second positioner PW configured to accurately positionthe substrate in accordance with certain parameters, and a projectionsystem (e.g., a refractive projection lens system) PS configured toproject a pattern imparted to the radiation beam B by patterning deviceMA onto a target portion C (e.g., comprising one or more dies) of thesubstrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, includingrefractive, reflective, catadioptric, magnetic, electromagnetic andelectrostatic optical systems, or any combination thereof, asappropriate for the exposure radiation being used, or for other factorssuch as the use of an immersion liquid or the use of a vacuum. Any useof the term “projection lens” herein may be considered as synonymouswith the more general term “projection system.”

In this embodiment, for example, the apparatus is of a transmissive type(e.g., employing a transmissive mask). Alternatively, the apparatus maybe of a reflective type (e.g., employing a programmable mirror array ofa type as referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables and, for example, two or more patterning devicetables. In such “multiple stage” machines the one or more additionaltables may be used in parallel, or preparatory steps may be carried outon one or more tables while one or more other tables are being used forexposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for examplebetween the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (which are commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam to have a desired uniformity and intensitydistribution in its cross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g., aninterferometric device, linear encoder, 2-D encoder or capacitivesensor), the substrate table WT can be moved accurately, e.g., so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioner PM and another position sensor (whichis not explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g., after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2 the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include one or more spincoaters SC to deposit resist layers, one or more developers DE todevelop exposed resist, one or more chill plates CH and/or one or morebake plates BK. A substrate handler, or robot, RO picks up substratesfrom input/output ports I/O1, I/O2, moves them between the differentprocess apparatuses and delivers then to the loading bay LB of thelithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU that is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure values of one or more propertiessuch as overlay errors between subsequent layers, line thicknesses,critical dimensions (CD), etc. If errors are detected, adjustments, forexample, can be made to exposures of subsequent substrates, especiallyif the inspection can be done soon and fast enough that other substratesof the same batch are still to be exposed. Also, already exposedsubstrates may be stripped and reworked to improve yield, or possibly bediscarded, thereby avoiding performing exposures on substrates that areknown to be faulty. In a case where only some target portions of asubstrate are faulty, further exposures can be performed only on thosetarget portions that are deemed to be non-faulty.

A metrology apparatus is used to determine the values or one or moreproperties of the substrates, and in particular, how the values of oneor more properties of different substrates or different layers of thesame substrate vary from layer to layer. The metrology apparatus may beintegrated into the lithographic apparatus LA or the lithocell LC or maybe a stand-alone device. To enable most rapid measurements, it isdesirable that the metrology apparatus measure one or more properties inthe exposed resist layer immediately after the exposure. However, thelatent image in the resist has a very low contrast, as in there is onlya very small difference in refractive index between the parts of theresist which have been exposed to radiation and those which have not—andnot all metrology apparatus have sufficient sensitivity to make usefulmeasurements of the latent image. Therefore measurements may be takenafter the post-exposure bake step (PEB) that is customarily the firststep carried out on exposed substrates and increases the contrastbetween exposed and unexposed parts of the resist. At this stage, theimage in the resist may be referred to as semi-latent. It is alsopossible to make measurements of the developed resist image, at whichpoint either the exposed or unexposed parts of the resist have beenremoved, or after a pattern transfer step such as etching. The latterpossibility limits the possibilities for rework of faulty substrates butmay still provide useful information.

An example metrology apparatus is schematically shown in FIG. 3a . Atarget T and diffracted rays of measurement radiation used to illuminatethe target are illustrated in more detail in FIG. 3b . The metrologyapparatus illustrated is of a type known as a dark field metrologyapparatus. The metrology apparatus may be a stand-alone device orincorporated in either the lithographic apparatus LA, e.g., at themeasurement station, or the lithographic cell LC. An optical axis, whichhas several branches throughout the apparatus, is represented by adotted line O. In this apparatus, radiation emitted by source 11 (e.g.,a xenon lamp) is directed onto substrate W via an optical element 15 byan optical system comprising lenses 12, 14 and objective lens 16. Theselenses are arranged in a double sequence of a 4F arrangement. Adifferent lens arrangement can be used, provided that it still providesa substrate image to a detector, and simultaneously allows for access ofan intermediate pupil-plane for spatial-frequency filtering. Therefore,the angular range at which the radiation is incident on the substratecan be selected by defining a spatial intensity distribution in a planethat presents the spatial spectrum of the substrate plane, here referredto as a (conjugate) pupil plane. In particular, this can be done byinserting an aperture plate 13 of suitable form between lenses 12 and14, in a plane which is a back-projected image of the objective lenspupil plane. In the example illustrated, aperture plate 13 has differentforms, labeled 13N and 13S, allowing different illumination modes to beselected. The illumination system in the examples of FIG. 3 forms anoff-axis illumination mode. In the first illumination mode, apertureplate 13N provides off-axis from a direction designated, for the sake ofdescription only, as ‘north’. In a second illumination mode, apertureplate 13S is used to provide similar illumination, but from an oppositedirection, labeled ‘south’. The rest of the pupil plane is desirablydark as any unnecessary radiation outside the desired illumination modewill interfere with the desired measurement signals. In otherembodiments, as discussed below with reference to FIGS. 4-8, apertureplates 13 of different form may be used, such as the aperture platelabeled 13H.

As shown in FIG. 3b , target T is placed with substrate W normal to theoptical axis O of objective lens 16. The substrate W may be supported bya support (not shown). A ray of measurement radiation I impinging ontarget T from an angle off the axis O gives rise to a zeroth order ray(solid line 0) and two first order rays (dot-chain line +1 and doubledot-chain line −1). It should be remembered that with an overfilledsmall target, these rays are just one of many parallel rays covering thearea of the substrate including metrology target T and other features.Since the aperture in plate 13 has a finite width (necessary to admit auseful quantity of radiation), the incident rays I will in fact occupy arange of angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown. Note that the grating pitches of the targetsand the illumination angles can be designed or adjusted so that thefirst order rays entering the objective lens are closely aligned withthe central optical axis. The rays illustrated in FIGS. 3a and 3b areshown somewhat off axis, purely to enable them to be more easilydistinguished in the diagram.

In the example of FIG. 3 at least the 0 and +1 orders diffracted by thetarget T on substrate W are collected by objective lens 16 and directedback through optical element 15. Returning to FIG. 3a , both the firstand second illumination modes are illustrated, by designatingdiametrically opposite apertures labeled as north (N) and south (S).When the incident ray I of measurement radiation is from the north sideof the optical axis, that is when the first illumination mode is appliedusing aperture plate 13N, the +1 diffracted rays, which are labeled+1(N), enter the objective lens 16. In contrast, when the secondillumination mode is applied using aperture plate 13S the −1 diffractedrays (labeled −1(S)) are the ones which enter the lens 16.

A beam splitter 17 divides the diffracted beams into two measurementbranches. In a first measurement branch, optical system 18 forms adiffraction spectrum (pupil plane image) of the target on first sensor19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for manymeasurement purposes such as reconstruction.

In the second measurement branch, optical system 20, 22 forms an imageof the target T on sensor 23 (e.g. a CCD or CMOS sensor). In the secondmeasurement branch, an aperture stop 21 is provided in a plane that isconjugate to the pupil-plane. Aperture stop 21 functions to block thezeroth order diffracted beam so that the image of the target formed onsensor 23 is formed only from the −1 or +1 first order beam. The imagescaptured by sensors 19 and 23 are output to processor PU which processesthe image, the function of which will depend on the particular type ofmeasurements being performed. Note that the term ‘image’ is used here ina broad sense. An image of the grating lines as such will not be formed,if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 3 are purely examples. In another embodiment, on-axis illuminationof the targets is used and an aperture stop with an off-axis aperture isused to pass substantially only one first order of diffracted radiationto the sensor. In yet other embodiments, 2^(nd), 3^(rd) and higher orderbeams (not shown in FIG. 3) can be used in measurements, instead of orin addition to the first order beams.

In order to make the measurement radiation adaptable to these differenttypes of measurement, the aperture plate 13 may comprise a number ofaperture patterns formed around a disc, which rotates to bring a desiredpattern into place. Note that aperture plate 13N or 13S can be used tomeasure gratings oriented in one direction (X or Y depending on theset-up). For measurement of an orthogonal grating, rotation of thetarget through 90° and 270° might be implemented.

FIG. 3c depicts an example of a (composite) target formed on asubstrate. The target in this example comprises four gratings orperiodic structures 25 a to 25 d positioned closely together so thatthey will all be within a measurement scene or measurement spot 24formed by the metrology radiation illumination beam of the metrologyapparatus. The four gratings or periodic structures thus are allsimultaneously illuminated and simultaneously imaged on sensors 19 and23. In an example dedicated to measurement of overlay, structures 25 ato 25 d are themselves composite periodic structures formed by overlyinggratings that are patterned in different layers of the semiconductordevice formed on substrate W. Structures 25 a to 25 d may havedifferently biased overlay offsets (deliberate mismatch between layers)in order to facilitate measurement of overlay between the layers inwhich the different parts of the composite gratings are formed. Suchtechniques are well known to the skilled person and will not bedescribed further. Structures 25 a to 25 d may also differ in theirorientation, as shown, so as to diffract incoming radiation in X and Ydirections. In one example, structures 25 a and 25 c are X-directiongratings with biases of the +d, −d, respectively. Structures 25 b and 25d are Y-direction gratings with offsets +d and −d respectively. Separateimages of these gratings can be identified in the image captured bysensor 23. This is only one example of a target. A target may comprisemore or fewer than four gratings, or only a single grating.

FIG. 3d shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 3c in the apparatus of FIG.3a . While the pupil plane image sensor 19 cannot resolve the differentindividual structures 25 a to 25 d, the image sensor 23 can do so. Thedark rectangle represents the field of the image on the sensor, withinwhich the illuminated spot 24 on the substrate is imaged into acorresponding circular area 26. Within this, rectangular areas 27 a to27 d represent the images of the small target structures 25 a to 25 d.If the targets are located in product areas, product features may alsobe visible in the periphery of this image field. Image processor andcontroller PU processes these images using pattern recognition toidentify the separate images 27 a to 27 d of structures 25 a to 25 d. Inthis way, the images do not have to be aligned very precisely at aspecific location within the sensor frame, which greatly improvesthroughput of the measuring apparatus as a whole.

Once the separate images of the periodic structures or gratings havebeen identified, the intensities of those individual images can bemeasured, e.g., by averaging or summing selected pixel intensity valueswithin the identified areas. Intensities and/or other properties of theimages can be compared with one another. These results can be combinedto measure different parameters of the lithographic process. Overlayperformance is an important example of such a parameter.

As mentioned in the introductory part of the description, measurementsof overlay and other asymmetries in target structures is difficult whenstructures are at the resolution of device features to be manufactured.This is because it is difficult to capture higher than zeroth orderdiffracted radiation. In an arrangement of the type depicted in FIGS.3a-3d for example, the angles of either or both of the +1 and −1diffracted orders become too high for both to be captured by theobjective lens 16 or these orders become evanescent (non-propagating).

Target asymmetry makes a contribution, albeit extremely small, to zerothorder reflected beams (i.e. specular reflected beams). Zeroth orderreflected beams are relatively easily captured by the objective lens 16.Interferometry can be used to measure the asymmetry contribution to thezeroth order reflected beams with high sensitivity, as well as otherparameters of interest. Embodiments based on this principle aredescribed below.

According to an embodiment, a metrology apparatus configured to measurea structure formed on a substrate by a lithographic process is provided.In an embodiment, the metrology apparatus is broadly similar to themetrology apparatus of FIG. 3 in the case where only the firstmeasurement branch (in which a detector is placed in a pupil plane) isprovided. It is not however necessary for the detection to take place inthe pupil plane. In other embodiments the detector is placed in theimage plane or in a plane between the image plane and the pupil plane.The metrology apparatus comprises an optical system (described belowwith reference to FIGS. 4 and 5) that focuses radiation onto thestructure and directs radiation after redirection by the structure to adetection system 102 (via arrow 100). Further details about exampledetection systems 102 are given below with reference to FIGS. 13-17. Theoptical system is configured such that the detection system 102 detectsa radiation intensity resulting from interference between radiation fromat least two different points in a pupil plane field distribution. Theinterference is such that a component of the detected radiationintensity containing information about a parameter of interest isenhanced relative to one or more other components of the detectedradiation intensity (due to at least partially destructive interferenceof radiation corresponding to the one or more other components). Theoptical system may introduce the required spatial coherence betweendifferent points in the pupil plane field distribution, so thefunctionality can be implemented using an incoherent radiation source.In an embodiment, the detected radiation intensity results from zerothorder reflection from the structure. The approach is therefore suitablefor measuring high resolution features (e.g. features at the resolutionof device structures to be manufactured).

The embodiments discussed with reference to FIGS. 4-7, 13, 14 and 16implement the above functionality using a form of common pathinterferometry, in which radiation that is split by a beam splitterfollows a common path in different senses before being interfered afterpassing through the beam splitter a second time. The parameter ofinterest in these embodiments is overlay, but the principle could beapplied to one or more other parameters of interest.

FIG. 4 depicts optical elements of an example metrology apparatusconfigured to provide an input radiation beam 34 to an optical unit 40(shown in FIGS. 5-7). A source 11 (e.g. an output end of an opticalfiber) provides a radiation beam that is passed through a lens systemcomprising lenses 12, 14A and 14B. The lenses 12, 14A and 14B correspondto lenses 12 and 14 shown in FIG. 3. Like the lenses 12 and 14 of FIG.3, the lenses 12, 14A and 14B may be arranged in a double sequence of a4F arrangement. A pupil plane in which a pupil plane field distributionis formed and is labeled 32. An image plane in which an image of thesource (e.g. the end of the optical fiber) is formed is labeled 35. Anaperture plate 13 is provided in a pupil plane 32. The aperture plate 13may take the form depicted by aperture plate 13H for example (viewedfrom above). The aperture plate 13 imparts a desired pupil plane fielddistribution to the input radiation 34 provided to a beam splitter 48and will be described in further detail below. The input radiation 34may optionally be polarized by a polarizer (e.g. linearly polarized).

As depicted in FIGS. 6, 7, 13 and 14, in embodiments the optical unit 40comprises a beam splitter 48. The beam splitter 48 splits the inputradiation beam 34 into a first radiation beam and a second radiationbeam. The optical unit 40 is part of an optical system (depicted in FIG.5) that directs the first radiation beam and the second radiation beamonto a substrate W and directs redirected radiation from the substrate Wto a detection system 102 (which may comprise one or more CCD or CMOSsensors for example) via the beam splitter 48 (via arrow 100). Thedetection system 102 may detect in a pupil plane. The detection system102 may thus record an intensity in a pupil plane field distribution ofa combination of the first radiation beam and the second radiation beamafter redirection by the substrate W. As will be described in furtherdetail below, the detection system 102 detects radiation resulting frominterference between the first radiation beam and the second radiationbeam. In an embodiment the interference is such that the first radiationbeam and the second radiation beam interfere more destructively (e.g.completely destructively) at the point of detection for reflections froma symmetric component of a target structure than for reflections from anasymmetric component of the target structure. A background signal thatdoes not contain information about asymmetry in a target structure isthereby removed or reduced. A portion of the signal that does containinformation about the asymmetry in the target structure is retained. Thesensitivity with which the asymmetry can be measured is therebyincreased. The interference between the first radiation beam and thesecond radiation beam comprises interference between different points ina pupil plane field distribution. In these embodiments, pairs of pointsin the pupil plane field distribution that are to interfere with eachother are arranged symmetrically about a common point (for pointsymmetry) or common axis (for mirror symmetry) of symmetry. When thepupil plane field distribution is perfectly symmetric about the commonpoint or axis of symmetry the pairs of points have the same amplitudeand can be made to interfere destructively by applying a 180 degreesphase shift between them. A symmetric background signal can thus beremoved efficiently and any deviation from symmetry can be detected withhigh sensitivity. FIG. 6 described below depicts an example in whichdifferent points in the pupil plane field distribution are interferedmirror symmetrically. FIG. 7 described below depicts an example in whichdifferent points in the pupil plane field distribution are interferedpoint symmetrically.

In an embodiment, reflected first radiation beam and reflected secondradiation beam reaching the detector result from zeroth order reflectionfrom a target structure on the substrate W. The approach is thereforesuitable for measuring high resolution features (e.g. features at theresolution of device structures to be manufactured).

In the embodiment of FIGS. 4-7, 13, 14 and 16, an optical system 60 issuch that the first radiation beam and the second radiation beampropagate in opposite directions around a common optical path comprisinga first branch 61 and a second branch 62. In the embodiment shown, thefirst branch 61 and the second branch 62 have optical elements in common(e.g. lenses 42A, 42B and 44) although the radiation propagates throughdifferent portions of these common optical elements in each branch. Thecommon optical path is common in the sense that the optical trajectoryof the first radiation beam and the optical trajectory of the secondradiation beam can be superimposed onto each other (within engineeringtolerances). The only difference between the optical trajectories of thefirst radiation beam and the second radiation beam in the common opticalpath is that the first radiation beam and second radiation beam travelin opposite directions. The common optical path is a closed opticalpath. The first radiation beam propagates from the beam splitter 48 tothe substrate W along the first branch 61 (downwards in the examplesshown) and from the substrate W back to the beam splitter 48 along thesecond branch 62 (upwards in the examples shown). The second radiationbeam propagates from the beam splitter 48 to the substrate W along thesecond branch 62 (downwards in the examples shown) and from thesubstrate W back to the beam splitter 48 along the first branch 61(upwards in the examples shown). The first radiation beam and the secondradiation beam are focused onto the same location on the substrate,forming an image on the substrate W (e.g. an image of the source 11). Aphase shift is applied to the first radiation beam relative to thesecond radiation beam to increase destructive interference between thefirst radiation beam and the second radiation beam at the detector 38(relative to the case where no phase shift is applied). In an embodimentthe phase shift is applied uniformly to the whole of the cross-sectionof the first radiation beam relative to the whole of the cross-sectionof the second radiation beam. In one particular class of embodiments,the phase shift is equal to 180 degrees. The phase shift is such as tocause the component of the detected radiation intensity containinginformation about the parameter of interest (e.g. overlay) to beenhanced by interference relative to the one or more other components ofthe detected radiation intensity.

Due to the common optical path of the first radiation beam and thesecond radiation beam, if the target structure from which the firstradiation beam and the second radiation beam is redirected is fullysymmetric (e.g. point symmetric or mirror symmetric), completedestructive interference can be achieved at the detection system 102 forall points in the pupil plane field distribution, in the case of anapplied phase difference of 180 degrees. Any asymmetry in the targetstructure, due to overlay for example, will cause incomplete destructiveinterference. The incomplete destructive interference provides a signalat the detection system 102 that can be used to obtain a measure of theasymmetry. The interferometry thus removes unwanted background signaland improves a sensitivity with which the asymmetry can be measured.

The extent to which background signal can be removed will depend onalignment accuracy of optical elements such as the beam splitter 48and/or optical imperfections. Imperfect alignment will lead to fringes(due to reflected beams from the first radiation beam and the secondradiation beam not lying exactly on top of each other or not propagatingin exactly the same direction). Imperfect optics will lead to incompletebackground suppression, for example if the beam splitter 48 does notprovide exactly 50/50 beam splitting.

In the example of FIG. 5, the first radiation beam and the secondradiation beam are both focused onto the substrate W by lenses 42A, 42Band 44. An image plane between lenses 42A and 42B is labeled 35. Thesubstrate W is also positioned in an image plane. A pupil plane betweenlenses 42B and 44 is labelled 32. Reflected radiation from the firstradiation beam and the second radiation beam is directed to the detector38 (see, e.g., FIG. 9), after passing a second time through the beamsplitter 48, via lenses 18A and 18B. An image plane between lenses 18Aand 18B is labeled 35.

In an embodiment, the first radiation beam and the second radiation beamare directed onto the substrate W symmetrically. The symmetry may resultin a pupil plane field distribution of the first radiation beam beingmirror symmetric or point symmetric with respect to a pupil plane fielddistribution of the second radiation beam (which is in the same plane asthe pupil plane field distribution of the first radiation beam) prior toredirection of the first radiation beam and the second radiation beamfrom the substrate W. The optical system performs at least one flip orrotation of the pupil plane field distribution of radiation propagatingin the first branch or the second branch such that the image from thefirst radiation beam and the image from the second radiation beam arerespectively formed by radiation having pupil plane field distributionsthat are mirror symmetric or point symmetric with respect to each other.

In the example of FIG. 6 the pupil plane field distribution of radiationpropagating in the first branch is flipped (reflected) such that theimage from the first radiation beam and the image from the secondradiation beam are respectively formed by radiation having pupil planefield distributions that are mirror symmetric with respect to eachother. In embodiments of this type an optical path length compensator 50may be provided to compensate for the additional optical path lengthintroduced by the flipping of the pupil plane field distribution. In theparticular example of FIG. 6, the pupil plane is flipped by pupil planefield distribution modifying unit 46 in the first branch 61. The opticalpath length compensator 50 is then positioned in the second branch 62.

The pupil plane field distribution modification unit 46 may beimplemented in various ways. In the configuration shown, any combinationof optical elements that achieves the desired function of changing thedirection of the radiation beam (from horizontal to down) and flippingthe pupil plane field distribution may be used. The functionality can beimplemented using two suitably oriented mirrors or a pentaprism forexample.

The optical path length compensator 50 may be implemented in variousways. Any combination of optical elements that achieves the desiredfunction of making the optical path length from beam splitter 48 to thetarget structure on the substrate W the same for the first radiationbeam and the second radiation beam (by compensating for the detourthrough the pupil plane field distribution modification unit 46) may beused. This is used to help ensure that the target structure is in theimage plane and therefore in focus (allowing optimal measurement of thetarget structure). In the particular example of FIG. 6, the optical pathlength compensator 50 comprises four mirrors. The optical path lengthcompensator 50 could alternatively be implemented using right angleprisms, or a combination of right angle prisms and mirrors. The opticalpath length compensator 50 can be fixed (e.g. perfectly matched to thepupil plane field distribution modification unit 46) or tunable inlength (for flexibility). In principle, a plate of glass could be used(because of the high index of refraction).

FIG. 7 depicts an implementation of symmetrically directing the firstradiation beam and the second radiation beam onto the substrate W. Incontrast to the embodiment of FIG. 6 in which mirror symmetry isachieved, the arrangement of FIG. 7 results in the pupil plane fielddistribution of the first radiation beam being point symmetric withrespect to the pupil plane field distribution of the second radiationbeam prior to redirection of the first radiation beam and the secondradiation beam by the substrate W. In the example of FIG. 7 this isachieved by modifying the arrangement of FIG. 6 to add an additionalflip (mirror reflection) in the first branch 61. In the example shown,the additional flip is implemented by a dove prism 80. In an alternativeembodiment, the additional flip is implemented using a roof top Amiciprism, for example in place of one of the mirrors of the optical pathlength compensator 50. Alternatively, the additional flip is provided inthe second branch 62. Alternatively, the effect can be achieved byrotation of the pupil plane field distributions, for example byimplementing −90 degrees rotation in one of the branches and a +90degrees rotation in the other branch. Point symmetry is desirablebecause it corresponds to interfering radiation beams that haveinteracted with the target from opposite directions. This may not benecessary for aligned grating targets where the symmetry of the targetsthemselves means that mirror symmetry in the pupil plane fielddistributions may be adequate. When the overlay target is not aligned,however, or when it is desired to measure product features, it may benecessary to use an embodiment such as that of FIG. 7 to help ensurethat the pupil plane field distributions are point symmetric.

The beam splitter 48 can be implemented in various ways. In the exampleshown a plate beam splitter is used. In other embodiments, a cube beamsplitter or a pellicle beam splitter is used. For maximum destructiveinterference, a 50/50 beam splitter is desirable.

When measuring asymmetry only, such as overlay only, a phase shift of180 degrees may be used. However, using another phase shift will meanincomplete suppression of the background signal. This may be beneficialwhere it is desired to obtain information from the background signal.Information about symmetrical properties of the target (e.g. criticaldimension) may be obtained for example. In an embodiment, the metrologyapparatus is configured so that the phase shift is selectivelycontrollable. The level of background signal can therefore be tuned asdesired or the measurement can be switched between a mode that issensitive predominantly to asymmetric properties and a mode that issensitive predominantly to symmetric properties. In an embodiment, thephase shift is arranged at least temporarily to be close to 180 degreesbut not exactly 180 degrees (e.g. 180 degrees plus or minus a shift of 1or more degrees, plus or minus 2 or more degrees, plus or minus 5 ormore degrees, plus or minus 10 or more degrees, or plus or minus 20 ormore degrees). Control of the phase shift may be implemented by suitableadaptation of the beam splitter 48 for example.

Alternatively or additionally, measurement of symmetric properties maybe achieved by providing an apparatus to selectively remove the beamsplitter 48 or to selectively replace the beam splitter 48 with adifferent component, such as a two sided mirror. Alternatively oradditionally, the beam splitter 48 may be configured to have a beamsplitting ratio other than 50/50 (which will result in incompletedestructive interference with respect to symmetric components of thetarget structure).

In the embodiments of FIGS. 6 and 7, a phase shift of 180 degreesbetween the reflected first radiation beam and the reflected secondradiation beam is provided by the different ways the two beams arereflected or transmitted through the beam splitter. In the particularexample shown the first radiation beam is output by reflection from oneside (the left side) of the beam splitter 48 and is directed to thedetector 38, after propagation around the common optical path, byreflection from the opposite side (the right side) of the beam splitter48. This involves two reflections (one internal and one external). Thesecond radiation beam, in contrast, is output by transmission throughthe beam splitter 48 and is directed to the detector 38, afterpropagation around the common optical path, by transmission through thebeam splitter 48 a second time. Thus, if the optical path lengths arethe same, the 180 degree phase shift introduced by the one externalreflection from beam splitter provides the desired 180 degrees phaseshift between the two radiation beams.

In an embodiment, the input radiation 34 to the beam splitter 48comprises a pupil plane field distribution in which a first region ofthe pupil plane field distribution has been removed to leave only asecond region of the pupil plane field distribution. In the embodimentof FIGS. 4-7, 13, 14 and 16, the first region is removed by apertureplate 13H. In an embodiment, the first region and the second region areoppositely oriented semicircles. This approach is desirable because itallows a maximum proportion of the radiation to contribute to thesymmetrical illumination of the substrate W. A full circular pupil planefield distribution is provided at lens 44. One half is provided by thefirst radiation beam and the other half is provided by the secondradiation beam. In an embodiment of this type, the flipping of the pupilplane field distribution may comprise a reflection about the straightedge of the semicircle of the first region of the pupil plane (FIG. 6)and/or a reflection about a line of mirror symmetry of the semicircle ofthe first region of the pupil plane (FIG. 7).

FIG. 6 depicts the pupil plane field distributions at various points inthe optical path between the input of the input radiation 34 to theoptical unit 40 and the output from the optical unit 40 of the radiationbeams after redirection by the substrate W. The pupil plane fielddistribution of the input radiation 34 at the point of entry into theoptical unit 40 is labelled 70 (as viewed from above). The arrowindicates the direction of propagation of the radiation (downwards inthis case). The circle, square and triangle are provided in the figure(they are not present in the actual pupil plane field distributions) toidentify reference parts of the pupil plane field distribution in orderto facilitate visual tracking of the orientation of the pupil planefield distribution through the optical system in the Figure.

As described above, the input radiation 34 is split by the beam splitterinto a first radiation beam and a second radiation beam.

The first radiation beam follows the first branch 61 and passes throughthe pupil plane field distribution modification unit 46 before exitingthe optical unit 40 downwards. The pupil plane field distribution atthis stage (as viewed from above) is labelled 71A. As can be seen, pupilplane field distribution 71A is a mirror image of pupil plane fielddistribution 70. The axis of mirror symmetry is the straight edge of thesemicircle. The first radiation beam passes through optics between theoptical unit 40 and the substrate W (the rest of the first branch 61) toform an image on the substrate W. The first radiation beam is thenredirected by the substrate W and propagates upwards along the secondbranch 62. The redirected first radiation beam passes through the opticsbetween the substrate W and the optical unit 40. The pupil plane fielddistribution of the redirected first radiation beam on entry to theoptical unit is labelled 71B (viewed from above). The optics between theoptical unit 40 and the substrate W leads to rearrangement of the pupilplane field distribution 71A in a point symmetric way to provide thepupil plane field distribution 71B. The redirected first radiation beampasses through the optical path length compensator 50 upwards and isoutput from the optical unit 40 after reflection from the beam splitter48. The pupil plane field distribution at this stage (viewedhorizontally from the left) is labeled 71C.

The second radiation beam propagates around the common optical path inthe opposite sense to the first radiation beam. The pupil plane fielddistribution of the second radiation beam after transmission through thebeam splitter 48 and propagation through the optical path lengthcompensator 50 is labelled 72A (viewed from above). Pupil plane fielddistribution 72A is essentially identical to pupil plane fielddistribution 70. The second radiation beam passes through optics betweenthe optical unit 40 and the substrate W (the rest of the second branch62) to form an image on the substrate W. The second radiation beam isthen redirected by the substrate W and propagates upwards along thefirst branch 61. The redirected second radiation beam passes through theoptics between the substrate W and the optical unit 40. The pupil planefield distribution of the redirected second radiation beam on entry tothe optical unit 40 is labelled 72B (viewed from above). The opticsbetween the optical unit 40 and the substrate W leads to rearrangementof the pupil plane field distribution 72A in a point symmetric way toprovide the pupil plane field distribution 72B. The redirected secondradiation beam passes through the pupil plane field distributionmodification unit 46 and is output from the optical unit 40 aftertransmission through the beam splitter 48 a second time. The pupil planefield distribution at this stage (viewed horizontally from the left) islabeled 72C.

FIG. 7 depicts the pupil plane field distribution at the same points asFIG. 6. The additional flip discussed above causes the pupil plane fielddistribution 71A to be point symmetric with respect to the pupil planefield distribution 72A instead of mirror symmetric.

Pupil plane field distributions 71C and 72C have the same orientationand lie exactly over each other (within engineering tolerances). Thiscauses radiation originating from pairs of points that are mirrorsymmetric or point symmetric with respect to each other in the pupilplane field distribution defined by the combination of distributions 71Band 72B in FIGS. 6 and 7 to interfere. Corresponding radiationintensities can then be detected at the detection system 102. In theschematic illustrations of FIGS. 6 and 7, the two triangles ofdistributions 71B and 72B will interfere, the two squares ofdistributions 71B and 72B will interfere, and the two circles ofdistributions 71B and 72B will interfere. If the pupil plane fielddistributions 71B and 72B are exactly the same as each other (becausethe target structure has not induced any asymmetry), destructiveinterference will cause the whole pupil plane field distribution to bedark. Because two copies of the half pupil are spatially overlapped itis not necessary to have spatial coherence throughout the pupil. Asdiscussed above, any asymmetry in the pupil plane field distributionwill cause incomplete destructive interference and thereby providebright regions. The bright regions can be detected by the detectionsystem 102 and provide information about asymmetry in the targetstructure.

In a further embodiment, a metrology apparatus is provided which uses anoptical pupil symmetrization (OPS) system to provide the destructiveinterference for the reflections from symmetric components of the targetstructure and the constructive interference for the reflections fromasymmetric components of the target structure (such as overlay). Detailsof how to implement an OPS system are provided in PCT Patent ApplicationPublication No. WO 2016/096310, which is hereby incorporated in itsentirety by reference.

In an embodiment, a metrology apparatus as described above withreference to FIGS. 4 and 5 is provided, except that the configuration ofFIG. 4 may not comprise the aperture plate 13H to remove the firstregion of the pupil field distribution and the optical unit 40 isconfigured as shown in FIG. 8. The optical unit 40 of FIG. 8 comprisesan OPS system. The optical unit 40 comprises a first beam splitter 83that splits the radiation beam 34 into a first radiation beam and asecond radiation beam. The optical unit 40 further comprises a secondbeam splitter 84 that recombines the first radiation beam and the secondradiation beam. The first radiation beam propagates along a firstoptical branch 81 between the first beam splitter 83 and the second beamsplitter 84. The second radiation beam propagates along a second opticalbranch 82 between the first beam splitter 83 and the second beamsplitter 84. The first optical branch 81 and the second optical branch82 flip or rotate a field distribution of the first radiation beamrelative to a field distribution of the second radiation beam about twoorthogonal axes. In the example of FIG. 8, the first radiation beam isflipped about a first axis in the first branch 81 using a first doveprism 85. The second radiation beam is flipped about a second axis,perpendicular to the first axis, in the second branch 82 using a seconddove prism 86. In an alternative implementation, optical elements areprovided that rotate the first radiation beam by −90 degrees in thefirst branch and rotate the second radiation beam by +90 degrees in thesecond branch. The optical path length along the first optical branch 81is essentially equal to the optical path length along the second opticalbranch 82.

The radiation beam passes through the first beam splitter 83 and thesecond beam splitter 84 before being redirected by the target structure(via optical system 60, which may be configured for example as shown inFIG. 5). The pupil plane field distribution of the radiation beam thatis focused onto the structure is point symmetric. The radiation beamthen additionally passes through the first beam splitter 83 and thesecond beam splitter 84 after redirection by the target structure (inthe opposite direction). This results in a first output 87 from thefirst beam splitter 83 being formed by the first radiation beam and thesecond radiation beam interfering destructively for reflections from asymmetric component of the target structure and interferingconstructively for reflections from an asymmetric component of thetarget structure. The first output 87 is therefore such that a componentof the detected radiation intensity containing information about theparameter of interest (e.g. overlay) is enhanced relative to one or moreother components (e.g. symmetric components).

Radiation can propagate through the OPS system of FIG. 8 via fourdifferent routes: 1) to the target structure via the first opticalbranch 81 and back to the first beam splitter 83 via the second opticalbranch 82, 2) to the target structure via the second optical branch 82and back to the first beam splitter 83 via the first optical branch 81,3) to the target structure via the first optical branch 81 and back tothe first beam splitter 83 via the first optical branch 81, and 4) tothe target structure via the second optical branch 82 and back to thefirst beam splitter 83 via the second optical branch 82. Routes 1 and 2considered together are similar to the common path interferometricembodiments discussed with reference to FIGS. 4-7. Routes 3 and 4considered together resemble a double Mach Zehnder interferometer. Bothpairs of routes provide a phase difference of 180 degrees in respect ofreflection from symmetric components of the target structure, therebyleading to destructive interference. Asymmetric components may interfereconstructively and thereby contribute to the detected signal via thefirst output 87.

FIG. 9 depicts an embodiment in which the OPS system of FIG. 8 ispositioned so that the radiation beam passes through only afterredirection by the target structure (and not before). In embodiments ofthis type other arrangements may be provided to introduce spatialcoherence in radiation incident on the structure and/or the source 11may be configured to output spatially coherent radiation. The metrologyapparatus in this case may be as described above with reference to FIGS.4 and 5, except that the configuration of FIG. 4 may not comprise theaperture plate 13H to remove the first region of the pupil fielddistribution, the optical unit 40 in FIG. 5 has a single beam splitter,and the OPS system of FIG. 9 is provided after the lens 18B shown inFIG. 5. In this embodiment, a first detector 38A detects radiationoutput from a first output 87 of the second beam splitter 84. A seconddetector 38B detects radiation output from a second output 88 of thesecond beam splitter 84. The OPS system in this case operates accordingto the principles of a Mach Zehnder interferometer. When the pathlengths are equal in the first optical branch 81 and the second opticalbranch 82 from the beam splitter 83, the first output 87 will be darkdue to destructive interference and the second output 88 will be brightdue to constructive interference. As in the embodiment of FIG. 8, doveprisms 85 and 86 flip the field distributions of the first radiationbeam and the second radiation so that the two copies of the pupil arepoint symmetric when they are interfered. In the first detector 38A, theradiation is interfered destructively and only the asymmetry signal(from reflection from asymmetric components of the target structure)remains. This causes a component of the detected radiation intensitycontaining information about a parameter of interest (e.g. overlay) tobe enhanced relative to other components. In the second detector 38B,the radiation is interfered constructively. This allows the seconddetector 38B to detect a radiation intensity in which the componentcontaining information about a parameter of interest (e.g. overlay) issuppressed relative to other components. The second detector 38B canthus be used to measure the symmetric part of the pupil for example.

If the interferometric systems described above with reference to FIGS.6-9 are perfectly balanced, measured radiation intensity approximatelyfollows a cosine curve as a function of target asymmetry (e.g. overlay).The symmetry of the cosine curve means the sign of the detectedasymmetry is ambiguous if only a single value of intensity is obtained.A given magnitude of asymmetry with a first sign (e.g. overlay in a +Xdirection) will give rise to the same intensity as the same magnitude ofasymmetry with the opposite sign (e.g. overlay in a −X direction). Oneapproach for dealing with this issue is to use metrology targets withnon-zero nominal asymmetry, e.g. metrology targets having an overlaybias. This approach involves special metrology targets and is relativelyinflexible and limited in terms of how much extra information isprovided. Approaches that provide increased flexibility by applyingoffsets in optical characteristics of radiation are described below.

In an embodiment, the optical system is configured to apply a pluralityof different offsets of an optical characteristic to radiation beforeand/or after redirection by the target structure. The different offsetsare applied in such a way that a corresponding plurality of differentoffsets are provided to reflected radiation derived from a first pointof a pupil plane field distribution relative to reflected radiationderived from a second point of the pupil plane field distribution. Adetection system 102 is provided that detects a corresponding pluralityof radiation intensities resulting from interference between thereflected radiation derived from the first point of the pupil planefield distribution and the reflected radiation derived from the secondpoint of the pupil plane field distribution. Each radiation intensitycorresponds to a different one of the plurality of different offsets.Sets of radiation intensities resulting from interference betweenreflected radiation from a plurality of different pairs of first andsecond points in the pupil plane field distribution may be detected.Each set of radiation intensities comprises a radiation intensity foreach of the plurality of different offsets. In an embodiment, each pairof points is positioned mirror symmetrically with respect to each otherabout the same line of mirror symmetry or point symmetrically withrespect to each other about the same symmetry point. In an embodiment,at least two of the plurality of radiation intensities corresponding tothe plurality of different offsets are measured at different times inthe same measurement branch (as discussed below with reference to FIGS.13-15 for example). This approach allows measurements to be made atmultiple offsets using a compact apparatus. In an embodiment, at leasttwo of the plurality of radiation intensities corresponding to theplurality of different offsets are measured simultaneously in differentmeasurement branches (as discussed below with reference to FIGS. 16 and17 for example). This approach allows measurements to be made quickly atmultiple offsets.

The approach may be applied to any of the embodiments discussed above,including embodiments in which radiation from different points in thepupil plane field distribution are interfered using common pathinterferometry based architectures (such as in FIGS. 6 and 7) and/or OPSbased architectures (such as in FIGS. 8 and 9). In one class ofembodiments, the different offsets comprise different offsets in phase.In another class of embodiments, the different offsets comprisedifferent offsets in amplitude. In another class of embodiments, thedifferent offsets comprise a combination of amplitude and phase offsets.

As discussed in detail above, the interference between the radiationfrom different points in the pupil plane field distribution may be suchthat a component of the detected radiation intensity containinginformation about the parameter of interest is enhanced relative to oneor more other components of the detected radiation intensity. Inessence, the interference causes contributions from the one or moreother components to at least partially cancel out by destructiveinterference. Applying an offset in phase or amplitude between radiationfrom the two different points adjusts the extent to which thecancellation occurs.

FIG. 10 depicts how a detected intensity I produced by the interferenceis expected to vary as a function of an applied phase bias 8 for threedifferent overlay values. Curve 111 corresponds to zero overlay, curve112 corresponds to a positive overlay, and curve 113 corresponds to anegative overlay. Each of the curves 111-113 takes the form cos(π+bias)with a decay for larger biases due to limited temporal coherence. Phaseasymmetry due to asymmetry in the target structure (e.g. overlay) shiftsthe curves to the left or right. The magnitude of the shift providesinformation about the magnitude of the asymmetry in the targetstructure. The direction of the shift provides information about thesign of the asymmetry in the target structure. In the particular exampleshown, positive overlay shifts the curve to the left (curve 112) andnegative overlay shifts the curve to the right (curve 113).

Making multiple measurements of the interference intensity at differentphase biases makes it possible to make a fit to the curve. An exampleplot of six such measurements is shown in FIG. 11. Fitting a curve tothese points would indicate that the overlay of the target structuremost closely matches the curve 112 in FIG. 10. Both the magnitude andsign of the target asymmetry can thus be obtained with high accuracy.The ability to match accurately to theoretically generated curves ofinterference intensity against applied amplitude and/or phase offsetprovides the basis more generally for determining more detailedinformation about the target structure being analyzed than would bepossible without the use of multiple applied offsets.

In an embodiment, the different offsets comprise at least one offset ina first sense (e.g. increasing an amplitude of radiation from a first oftwo points contributing to interference relative to the second of thetwo points, or increasing a phase angle of radiation from the firstpoint relative to the second point) and at least one offset in a secondsense (e.g. decreasing an amplitude of radiation from the first pointrelative to the second point, or decreasing a phase angle of radiationfrom the first point relative to the second point), opposite to thefirst sense. In an embodiment, the two offsets are essentially equal insize to each other but opposite in sign. This approach allows thederivative of the curve of intensity against offset to be obtainedaccurately with a minimum of two offsets being needed.

FIG. 12 depicts an example approach in which two measurements have beenmade at points 114 and 115. Fitting a straight line 116 to the twopoints yields an estimate of the derivative of the curve at zero offset,which identifies that the target structure being measured most closelyresembles the target structure corresponding to curve 112 (i.e. positiveoverlay). The derivative would have the opposite sign for negativeoverlay (curve 113), and thereby provides a sensitive measure of thesign of the detected asymmetry (in this case overlay) in the targetstructure. The derivative also provides information about the magnitudeof the asymmetry (e.g. overlay). A larger derivative corresponds to moreasymmetry (e.g. overlay).

The different offsets can be applied in various ways. Some examples aredescribed below.

In one class of embodiments, the different offsets are at leastpartially defined by a polarization-dependent optical element 131. Thepolarization-dependent optical element 131 modifies an amplitude orphase of radiation passing through the polarization-dependent opticalelement 131 in dependence on the polarization of the radiation. In suchembodiments, radiation from or forming the first point of the pupilplane field distribution passes through the polarization-dependentoptical element 131 with a different polarization than radiation from orforming the second point of the pupil plane field distribution. Thepolarization-dependent optical element 131 can thus modify the radiationfrom the first point differently from the radiation from the secondpoint and thereby apply the desired offset.

FIG. 13 depicts an example implementation in which the optical unit 40of FIG. 6 is modified to include a polarization-dependent opticalelement 131 in the first branch 61. The polarization-dependent opticalelement 131 could be positioned in the second branch 62. If the targetstructure at least partly converts the polarization of the radiationupon reflection (e.g. from a grating) and information about theparameter of interest is contained within the direction dependence ofthis polarization conversion, then polarization-dependent opticalelement 131 can apply an offset because the polarization properties ofradiation propagating clockwise around the loop of FIG. 13 (i.e. downthe second branch 62 and up the first branch 61) are potentiallydifferent at the polarization-dependent optical element 131 from thepolarization properties of radiation propagating anticlockwise (i.e.down the first branch 61 and up the second branch 62). This is becausethe clockwise radiation encounters the polarization-dependent opticalelement 131 after redirection by the target structure, while theanticlockwise radiation encounters the polarization-dependent opticalelement 131 before redirection by the target structure. As an example,when the input radiation 34 is purely X-polarized, Y-polarized radiationwill only be present in redirected radiation (due to the polarizationconversion properties of the target structure). If thepolarization-dependent optical element 131 retards Y-polarized radiationrelative to X-polarized radiation this will thus affect only theclockwise traveling radiation, which encounters thepolarization-dependent optical element 131 after redirection by thetarget structure (where a Y-polarization is present). The anticlockwisetravelling radiation only passes through the polarization-dependentoptical element 131 before redirection by the target, when noY-polarization is present, and is not therefore retarded. This leads toan offset between the radiation in the clockwise and anticlockwisepropagation directions. When the pupil plane field distributions aresubsequently superimposed to cause interference (see distributions 71Cand 72C in FIGS. 6 and 7), the applied offset constitutes an offset ofan optical property (phase or amplitude) provided to reflected radiationderived from a first point of a pupil plane field distribution (e.g.from a point in the portion of the pupil plane field distributionlabelled 71A defining the radiation incident onto the target structure,provided by anticlockwise radiation) relative to reflected radiationderived from a second point of the pupil plane field distribution (e.g.from a point in the portion of the pupil plane field distributionlabelled 72A that is symmetrically opposite to the abovementioned pointin the portion of the pupil plane field distribution labelled 71A,provided by the clockwise radiation). In the case where the offsetcomprises an offset in phase, the polarization-dependent optical element131 may comprise a retarder as in the example above. In the case wherethe offset comprises an offset in amplitude, the polarization-dependentoptical element 131 may comprise a diattenuator (also known as lineardichroic optics, polarization dependent loss optics, or a weakpolarizer).

In an embodiment, the retarder is a variable retarder. Using a variableretarder allows the phase offset to be finely tuned easily and/or to bechanged efficiently to substantially different values in order to applya plurality of different phase offsets. The variable retarder maycomprise a Soleil Babinet compensator, or a Pockels cell. A Pockels cellcan be controlled on a nanosecond timescale. The use of apolarization-dependent optical element 131 positioned within the firstbranch 61 and/or second branch 62 can be implemented with minimal impactto alignment, due to the lack of a need for moving parts.

In the specific example of X-polarized input, the clockwise pathX-polarized output X_(out) and Y-polarized output Y_(out), after passingthrough a polarizer 120, in the measurement branch 150 are given asfollows (with contributions on the right of the equation being asfollows, in order from right to left: input, beam splitter 48, targetstructure, retarder in polarization-dependent optical element 131,diattenuator in polarization-dependent optical element 131, beamsplitter 48, polarizer 120):

$\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix} = {{{{{{\begin{bmatrix}0 & 0 \\0 & 1\end{bmatrix}\begin{bmatrix}T_{x} & 0 \\0 & T_{y}\end{bmatrix}}\begin{bmatrix}a_{x} & 0 \\0 & a_{y}\end{bmatrix}}\begin{bmatrix}e^{i\; \phi_{x}} & 0 \\0 & e^{i\; \phi_{y}}\end{bmatrix}}\begin{bmatrix}R_{XX}^{+ k} & R_{XY}^{+ k} \\R_{YX}^{+ k} & R_{YY}^{+ k}\end{bmatrix}}\begin{bmatrix}T_{x} & 0 \\0 & T_{y}\end{bmatrix}}{\quad{{\begin{bmatrix}1 \\0\end{bmatrix}\mspace{79mu}\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix}} = \begin{bmatrix}0 \\{T_{y}a_{y}e^{i\; \phi_{y}}R_{YX}^{+ k}T_{x}}\end{bmatrix}}}}$

Outputs from the anticlockwise path after passing through the polarizer120 in the measurement branch 150 are given as follows (withcontributions on the right of the equation being as follows, in orderfrom right to left: input, beam splitter 48, diattenuator inpolarization-dependent optical element 131, retarder inpolarization-dependent optical element 131, target structure, beamsplitter 48, polarizer 120):

$\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix} = {{{{{\begin{bmatrix}0 & 0 \\0 & 1\end{bmatrix}\begin{bmatrix}R_{x} & 0 \\0 & R_{y}\end{bmatrix}}\begin{bmatrix}R_{XX}^{- k} & R_{XY}^{- k} \\R_{YX}^{- k} & R_{YY}^{- k}\end{bmatrix}}\begin{bmatrix}e^{i\; \phi_{x}} & 0 \\0 & e^{i\; \phi_{y}}\end{bmatrix}}\begin{bmatrix}a_{x} & 0 \\0 & a_{y}\end{bmatrix}}{\quad{{{\begin{bmatrix}{R_{x}e^{i\; \pi}} & 0 \\0 & {R_{y}e^{i\; \pi}}\end{bmatrix}\begin{bmatrix}1 \\0\end{bmatrix}}\mspace{79mu}\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix}} = \begin{bmatrix}0 \\{R_{y}R_{YX}^{- k}e^{i{({\phi_{x} + \pi})}}a_{x}R_{x}}\end{bmatrix}}}}$

The intensity from interference of the recombined radiation is given asfollows:

I _(interferometry)=(T _(y) a _(y) e ^(iφ) ^(y) R _(YX) ^(+k) T _(x) +R_(y) R _(YX) ^(−k) e ^(i(φ) ^(x) ^(+π)) a _(x) R _(x))²

In the embodiments discussed above, the beam splitter 48 has been anon-polarizing beam splitter. This may be appropriate in someembodiments because the asymmetry of interest (e.g. asymmetry due tooverlay) is normally an asymmetry between R_(XY) ^(+k) and R_(XY) ^(−k)or between R_(YX) ^(+k) and R_(YX) ^(−k) (i.e. an asymmetry betweenreflection from opposite positions in the pupil plane fielddistribution, −k and +k, involving the same change in polarization fromX to Y or from Y to X for both reflections). Replacing the beam splitter48 with a polarizing beam splitter in arrangements such as thosedepicted in FIGS. 6 and 7 would normally lead to a measurement of R_(YX)^(+k) vs R_(XY) ^(−k) or R_(XY) ^(+k) vs R_(YX) ^(−k), which would notbe sensitive to asymmetry due to overlay due to reciprocity/timereversal reasons.

A polarizing beam splitter can be used, however, if combined with a λ/2plate 122 as depicted in FIG. 14. The optical unit 40 of FIG. 14 is thesame as the optical unit 40 depicted in FIG. 13 except for thefollowing. The beam splitter 48 has been replaced by a polarizing beamsplitter 48A. A λ/2 plate 122 oriented at 45° has been provided in thefirst branch 61 instead of the polarization-dependent optical element131 (the λ/2 plate 122 oriented at 45° could be provided in the secondbranch 62 with the polarization-dependent optical element 131 omittedfrom the first branch 61). A variable retarder 131A and a further λ/2plate 131B have been provided in the measurement branch 150 downstreamfrom the polarizing beam splitter 48A.

The input radiation 34 in this embodiment has a polarization at 45°relative to the polarizing beam splitter 48A. This can be achieved by apolarizer, or a combination of a polarizer and a λ/2 plate at 45°. Thepolarizing beam splitter 48A will transmit 50% of the input radiation 34and reflect 50% of the input radiation 34. In an example, X-polarizedradiation is transmitted and Y-polarized radiation is reflected. The λ/2plate 122 causes the Y-polarized radiation in this example to beconverted to X-polarized radiation such that both the first branch 61and the second branch 62 illuminate the target structure withX-polarized radiation. After interaction with the target structure, aportion of the radiation containing information about asymmetry (e.g.due to overlay) will be converted into Y-polarized radiation. In theclockwise direction, this Y-polarized radiation is converted intoX-polarized radiation by the λ/2 plate 122, which is subsequentlytransmitted through the polarizing beam splitter 48A into the outputbranch. In the anticlockwise direction, the Y-polarized radiation fromthe target structure is reflected by the polarizing beam splitter 48Aand also enters the output branch. Thus, radiation that has undergonepolarization conversion at the target propagates towards the detectionsystem along the output branch while radiation that has not beenconverted does not.

In the measurement branch 150 immediately downstream from the polarizedbeam splitter 48A the radiation from clockwise propagation in the loopformed by the first branch 61 and second branch 62 has X polarization.Radiation from anticlockwise propagation has Y polarization. The furtherλ/2 plate 131B is oriented at 22.5° to convert the X-polarized radiationto −45° polarization and convert the Y-polarized radiation to +45°polarization. The polarizer 120 (or, alternatively, a further polarizingbeam splitter) is configured to project the −45° and +45° polarizationsonto the X and/or Y axis, such that the radiation from the clockwisepropagation can be made to interfere with the radiation from theanticlockwise propagation. The λ/2 plate 131B could also be left out ifthe polarizer 120 is oriented at +45°.

The above-described use of a polarizing beam splitter 48A opens up analternative range of approaches for applying phase or amplitude offsets.In the example shown, a variable retarder 131A is positioned in themeasurement branch 150. The radiation from the clockwise propagation hasorthogonal polarization to the radiation from the anticlockwisepropagation when passing through the variable retarder 131A, therebyallowing a phase offset to be applied to the radiation prior tointerference. There are several options for applying amplitude offsets.For example, the λ/2 plate 131B in the measurement branch 150 could berotated, resulting in the projection of the radiation onto the X and/orY axes being at a ratio other than 50/50. The variable retarder 131A andthe λ/2 plate 131B thus provide equivalent functionality to thepolarization-dependent optical element 131 discussed above withreference to FIG. 13. In other embodiments, an amplitude offset iscontrolled by rotating the polarizer 120 instead of, or in addition to,rotating the λ/2 plate 131B. Alternatively or additionally, thepolarization of the input radiation 34 could be tuned prior to reachingthe polarizing beam splitter 48A, which would yield a splitting ratioother than 50:50 at the first reflection from the polarizing beamsplitter 48A. The tuning of the polarization of the input radiation 34could be achieved by rotation of a λ/2 plate 161 and/or polarizer 162upstream of the polarizing beam splitter 48A. Thus, different offsetscan be at least partially defined by different angles between thepolarizing beam splitter 48A and either or both of a retarder (e.g. aλ/2 plate 161) and a polarizer 162.

Clockwise propagation through the loop formed by the first branch 61 andthe second branch 62 in the example of FIG. 14 is given as follows (withcontributions on the right of the equation being as follows, in orderfrom right to left: input, polarizing beam splitter 48A, targetstructure, λ/2 plate 122, polarizing beam splitter 48A):

$\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix} = {{{{{{\begin{bmatrix}1 & 0 \\0 & 0\end{bmatrix}\begin{bmatrix}0 & e^{{- i}\; {\pi/2}} \\e^{{- i}\; {\pi/2}} & 0\end{bmatrix}}\begin{bmatrix}R_{XX}^{+ k} & R_{XY}^{+ k} \\R_{YX}^{+ k} & R_{YY}^{+ k}\end{bmatrix}}\begin{bmatrix}1 & 0 \\0 & 0\end{bmatrix}}\begin{bmatrix}1 \\1\end{bmatrix}}\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix}} = \begin{bmatrix}{R_{YX}^{+ k}e^{{- i}\; {\pi/2}}} \\0\end{bmatrix}}$

Anticlockwise propagation is given as follows (with contributions on theright of the equation being as follows, in order from right to left:input, polarizing beam splitter 48A, λ/2 plate 122, target structure,polarizing beam splitter 48A):

$\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix} = {{{{{{\begin{bmatrix}0 & 0 \\0 & e^{i\; {\pi/2}}\end{bmatrix}\begin{bmatrix}R_{XX}^{- k} & R_{XY}^{- k} \\R_{YX}^{- k} & R_{YY}^{- k}\end{bmatrix}}\begin{bmatrix}0 & e^{{- i}\; {\pi/2}} \\e^{{- i}\; {\pi/2}} & 0\end{bmatrix}}\begin{bmatrix}0 & 0 \\0 & e^{i\; {\pi/2}}\end{bmatrix}}\begin{bmatrix}1 \\1\end{bmatrix}}\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix}} = \begin{bmatrix}0 \\{R_{YX}^{- k}e^{i\; {\pi/2}}}\end{bmatrix}}$

In the measurement branch 150, output from radiation from clockwisepropagation is given as follows (with contributions on the right of theequation being as follows, in order from right to left: input fromclockwise contribution, variable retarder 131A, λ/2 plate 131B,polarizer 120):

$\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix} = {{\begin{bmatrix}1 & 0 \\0 & 0\end{bmatrix}{{{{e^{{- i}\; {\pi/2}}\begin{bmatrix}{\cos ( {2\alpha} )} & {\sin ( {2\alpha} )} \\{\sin ( {2\alpha} )} & {- {\cos ( {2\alpha} )}}\end{bmatrix}}\begin{bmatrix}e^{i\; \phi_{x}} & 0 \\0 & e^{i\; \phi_{y}}\end{bmatrix}}\begin{bmatrix}{R_{YX}^{+ k}e^{{- i}\; {\pi/2}}} \\0\end{bmatrix}}\mspace{79mu}\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix}}} = \begin{bmatrix}{{\cos ( {2\alpha} )}R_{YX}^{+ k}e^{i\; \phi_{x}}e^{{- i}\; \pi}} \\0\end{bmatrix}}$

In the measurement branch 150, output from radiation from anticlockwisepropagation is given as follows (with contributions on the right of theequation being as follows, in order from right to left: input fromanticlockwise contribution, variable retarder 131A, λ/2 plate 131B,polarizer 120):

$\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix} = {{\begin{bmatrix}1 & 0 \\0 & 0\end{bmatrix}{{{{e^{{- i}\; {\pi/2}}\begin{bmatrix}{\cos ( {2\alpha} )} & {\sin ( {2\alpha} )} \\{\sin ( {2\alpha} )} & {- {\cos ( {2\alpha} )}}\end{bmatrix}}\begin{bmatrix}e^{i\; \phi_{x}} & 0 \\0 & e^{i\; \phi_{y}}\end{bmatrix}}\begin{bmatrix}0 \\{R_{YX}^{- k}e^{i\; {\pi/2}}}\end{bmatrix}}\mspace{79mu}\begin{bmatrix}X_{out} \\Y_{out}\end{bmatrix}}} = \begin{bmatrix}{{\sin ( {2\alpha} )}R_{YX}^{- k}e^{i\; \phi_{y}}} \\0\end{bmatrix}}$

The intensity from interference of the recombined radiation is given asfollows:

I _(interferometry)=(cos(2α)R _(YX) ^(+k) e ^(iφ) ^(x) e ^(−π)+sin(2α)R_(YX) ^(−k) e ^(iφ) ^(y) )².

Thus, different phase offsets can be applied by setting differentretardation amounts in the variable retarder 131A, which define thephases φ_(x) and φ_(y). Different amplitude offsets can be applied bysetting different values for the orientation of the λ/2 plate 131B(which defines the angle α).

The above embodiments also show that different offsets can be at leastpartially defined by providing different splitting ratios of a beamsplitter 48A. In the above example the different splitting ratio wasachieved by rotating the polarization of radiation incident onto apolarizing beam splitter 48A. Several other possibilities exist. Forexample, varying phase and amplitude offsets can be applied by moving agradient beam splitter.

FIG. 15 depicts a further approach for applying the phase and/oramplitude offsets based on the interferometric principles of the OPSsystem discussed above with reference to FIG. 9. As in the embodiment ofFIG. 9, and in contrast to the examples of FIGS. 13 and 14, in theembodiment of FIG. 15 the splitting and recombination of radiation tointerfere radiation from different points in the pupil plane fielddistribution is performed entirely after redirection of the radiation bythe target structure. The embodiment of FIG. 15 is similar to theembodiment of FIG. 9 except that functionality corresponding to the MachZehnder interferometer functionality of the first and second opticalbranches 81 and 82 of FIG. 9 is implemented by a self-referencinginterferometer 153. Thus, the embodiment of FIG. 15 may be implementedin a metrology apparatus as described above with reference to FIGS. 4and 5, except that the configuration of FIG. 4 may not comprise theaperture plate 13H to remove the first region of the pupil fielddistribution, the source 11 is configured to output spatially coherentradiation, the optical unit 40 in FIG. 5 has a single beam splitter, andthe components shown in FIG. 15 are provided after the lens 18B shown inFIG. 5.

The self-referencing interferometer 153 comprises a compound prism thatcan be interpreted as a Mach Zehnder interferometer with polarizing beamsplitters. In one arm of the self-referencing interferometer 153, thepupil plane field distribution and polarization are rotated by +90degrees. In the other arm, the pupil plane field distribution andpolarization are rotated by −90 degrees. The rotation is achieved bymultiple reflections. The output of the self-referencing interferometer153 comprises two superimposed copies of the pupil plane fielddistribution, rotated with respect to each other so that they are pointsymmetric. The two copies have orthogonal polarization relative to eachother.

In the embodiment of FIG. 15, a polarizer 151 is provided that iscrossed with respect to the polarization of the input radiation 34 intothe optical system 40. The polarizer thus removes radiation that isco-polarized with respect to the input radiation 34 into the opticalsystem 40. This may be useful for example where the parameter ofinterest is overlay, because information about overlay is expected to bepresent predominantly in radiation that is cross-polarized with respectto the input radiation 34 into the optical system 40. For otherparameters of interest, information may be present in the co-polarizedradiation and the polarizer 151 may be omitted in this case. A λ/2 plate152 is positioned downstream from the polarizer 151 (where provided). Inthis embodiment, the λ/2 plate 152 is oriented at 22.5° to rotate thepolarization of the radiation passing through the λ/2 plate 152 by 45°.The radiation then passes through the self-referencing interferometer153 and is output to a measurement branch 150. The measurement branch150 comprises a variable retarder 131A. The variable retarder 131A setsa phase difference between the two orthogonal polarizations. This can beused to compensate for unwanted path length differences in theself-referencing interferometer 153 or, as described below, to applymultiple different phase offsets. The measurement branch 150 furthercomprises a λ/2 plate 131B configured to rotate the polarization of thetwo copies by 45° (while maintaining the orthogonality of thepolarizations). The output from the λ/2 plate 131B enters a polarizingbeam splitter 157. The polarizing beam splitter 157 decomposes bothcopies of the pupil plane field distribution into two orthogonalpolarization components, one of which is transmitted and the otherreflected. The result is that the two copies of the pupil plane fielddistribution are made to interfere with each other. The interference ismeasured on either or both of the two detectors 158 and 159 (asdescribed above with reference to FIG. 9 for similarly configureddetectors 38A and 38B). For example, in detector 158, the radiation maybe interfered destructively such that only the asymmetry signal (fromreflection from asymmetric components of the target structure) remains(disregarding the effect of offsets). This causes a component of thedetected radiation intensity containing information about a parameter ofinterest (e.g. overlay) to be enhanced relative to other components. Inthe detector 158, the radiation may be interfered constructively. Thisallows the detector 158 to detect a radiation intensity in which thecomponent containing information about a parameter of interest (e.g.overlay) is suppressed relative to other components. The detector 158can thus be used to measure the symmetric part of the pupil for example.The information about the parameter of interest (e.g. overlay) couldtherefore be obtained using only one of the two detectors 158,159, so itis not essential to provide both of the detectors 158,159.

The arrangement of FIG. 15 can be used to apply amplitude offsets. In anembodiment, the λ/2 plate 131B is made rotatable. Rotation of the λ/2plate 131B away from the nominal 22.5° angle causes the splitting ratioof the polarizing beam splitter 157 to deviate from 50/50, which appliesan amplitude offset to one copy of pupil plane field distributionrelative to the other pupil plane field distribution, and thereby anamplitude offset between radiation from interfering pairs of points ofthe pupil plane field distribution detected by one of the detectors158,159. Alternatively or additionally, the λ/2 plate 152 can be rotatedto change the splitting ratio at the polarizing beam splitter in theself-referencing interferometer 153. The change in the splitting ratioat the polarizing beam splitter in the self-referencing interferometer153 will also apply an amplitude offset.

The arrangement of FIG. 15 can be used to apply phase offsets. In anembodiment, the variable retarder 131A is used to set a phase differencebetween the two copies of the pupil plane field distribution that arepolarized orthogonally relative to each other as they pass through thevariable retarder 131A. The variable retarder 131A and the λ/2 plate131B thus provide equivalent functionality to the variable retarder 131Aand the λ/2 plate 131B of FIG. 14 and to the polarization-dependentoptical element 131 of FIG. 13.

In an embodiment of FIG. 15, element 131A is a fixed retarder plate inwhich half of the pupil gets λ/4 retardation at 0° and the other halfhas no retardation. Further in the embodiment of FIG. 15, the polarizingelement 157 is a Wollaston prism. This embodiment of FIG. 15 facilitatesmeasurement of a parameter of interest in a single measurement stepwhich comprises mixing 4 representatives of the pupils.

In a further embodiment of FIG. 15, a self-referencing interferometer isused in combination with a Wollaston prism. In this embodiment of FIG.15, only 2 representative pupils are needed in order to calculate aparameter of interest.

Various arrangements for applying phase and/or amplitude offsets havebeen discussed above with reference to FIGS. 13-15. Multiple differentphase and/or amplitude offsets can be applied at different times bychanging settings of elements such as the variable retarder 131A and/orλ/2 plate 131B of FIGS. 14 and 15, or the polarization-dependent opticalelement 131 of FIG. 13. FIGS. 16 and 17 depict example implementationswhich allow measurements at multiple different amplitude and/or phaseoffsets to be achieved in parallel.

FIG. 16 depicts a detection system 102 configured to make measurementsat multiple different amplitude and/or phase offsets in parallel using aconfiguration of the type depicted in FIG. 14 except that thefunctionality provided by the variable retarder 131A, the λ/2 plate131B, and the polarizer 120 in the measurement branch 150 has beenprovided separately in a plurality of measurement branches 150A-D. Fourbeam splitters 142A-D are provided for distributing radiation outputfrom the polarized beam splitter 48A (arrow 100) simultaneously betweenthe four different measurement branches 150A-D. Within each measurementbranch 150A-D, a variable retarder 131AA-AD and a λ/2 plate 131BA-BD areprovided to perform the functionality of the variable retarder 131A andthe λ/2 plate 131B of FIG. 14. In this embodiment, rather than beingtuned differently at different moments in time to provide a plurality ofdifferent amplitude and/or phase offsets, the variable retarder 131AA-ADand λ/2 plate 131BA-BD in each measurement branch 150A-D is tuned toprovide a different amplitude and/or phase offset relative to each ofthe other measurement branches 150A-D. The functionality of thepolarizer 120 of FIG. 14 is provided in this embodiment by a polarizingbeam splitter 145A-D in each measurement branch 150A-D. Outputs from thepolarizing beam splitter 145A-D in each measurement branch 150A-D aredetected by at least one detector 146A-D, 147A-D that detects aradiation intensity corresponding to the interference between radiationfrom different points in the pupil plane field distribution, with aparticular amplitude and/or phase offset being applied that is differentfor each measurement branch 150A-D.

FIG. 17 depicts a detection system 102 configured to make measurementsat multiple different amplitude and/or phase offsets in parallel using aconfiguration of the type depicted in FIG. 15 except that thefunctionality provided by the variable retarder 131A, the λ/2 plate131B, the polarizing beam splitter 157, and the detectors 158 and 159 inthe measurement branch 150 has been provided separately in a pluralityof measurement branches 150A-D. Four beam splitters 142A-D are providedfor distributing radiation output from the self-referencinginterferometer 153 simultaneously between the four different measurementbranches 150A-D. Within each measurement branch 150A-D, a variableretarder 131AA-AD and a λ/2 plate 131BA-BD are provided to perform thefunctionality of the variable retarder 131A and the λ/2 plate 131B ofFIG. 15. In this embodiment, rather than being tuned differently atdifferent moments in time to provide a plurality of different amplitudeand/or phase offsets, the variable retarder 131AA-AD and the λ/2 plate131BA-BD in each measurement branch 150A-D is tuned to provide adifferent amplitude and/or phase offset relative to each of the othermeasurement branches 150A-D. Outputs from the polarizing beam splitter157A-D in each measurement branch 150A-D are detected by at least onedetector 157A-D, 158A-D that detects a radiation intensity correspondingto the interference between radiation from different points in the pupilplane field distribution, with a particular amplitude and/or phaseoffset being applied that is different for each measurement branch150A-D.

The above embodiments may be particularly usefully applied to measuringasymmetry in a target structure comprising a layered structure having afirst component in a first layer and a second component in a secondlayer, in the case where a separation between the first layer and thesecond layer is greater than λ/20, where A is a wavelength of the inputradiation beam. This may be the case for example where the method isapplied to a target structure after a lithographic development step butprior to a subsequent etching step. The increased sensitivity toasymmetry means that asymmetry (e.g. overlay between the first componentand the second component) can be measured for high resolution structureseven in cases such as these where the contribution to zeroth orderreflection is expected to be extremely small (due to the largeseparation between the layers). Additionally or alternatively,measurement times can be reduced significantly.

The concepts disclosed herein may find utility beyond post-lithographymeasurement of structures for monitoring purposes. For example, such adetector architecture may be used in future alignment sensor conceptsthat are based on pupil plane detection, used in lithographicapparatuses for aligning the substrate during the patterning process.

The targets described above may be metrology targets specificallydesigned and formed for the purposes of measurement. However, theability to measure high resolution targets means the embodiments mayalso be applied to targets that are functional parts of devices formedon the substrate. Many devices have regular, grating-like structures.The terms ‘target grating’ and ‘target’ as used herein do not requirethat the structure has been provided specifically for the measurementbeing performed.

The metrology apparatus can be used in a lithographic system, such asthe lithographic cell LC discussed above with reference to FIG. 2. Thelithographic system comprises a lithographic apparatus LA that performsa lithographic process. The lithographic apparatus may be configured touse the result of a measurement by the metrology apparatus of astructure formed by the lithographic process when performing asubsequently lithographic process, for example to improve the subsequentlithographic process.

An embodiment may include a computer program containing one or moresequences of machine-readable instructions describing methods ofmeasuring targets on a structures and/or analyzing measurements toobtain information about a lithographic process. There may also beprovided a data storage medium (e.g., semiconductor memory, magnetic oroptical disk) having such a computer program stored therein. Where anexisting lithography or metrology apparatus is already in productionand/or in use, an embodiment of the invention can be implemented by theprovision of updated computer program products for causing a processorto perform all or part of methods described herein.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that an embodiment of the invention may be used inother applications, for example imprint lithography, and where thecontext allows, is not limited to optical lithography. In imprintlithography a topography in a patterning device defines the patterncreated on a substrate. The topography of the patterning device may bepressed into a layer of resist supplied to the substrate whereupon theresist is cured by applying electromagnetic radiation, heat, pressure ora combination thereof. The patterning device is moved out of the resistleaving a pattern in it after the resist is cured.

Further embodiments are described in the below numbered clauses:

1. A metrology apparatus for measuring a structure formed on a substrateto determine a parameter of interest, the metrology apparatuscomprising:

an optical system configured to focus radiation onto the structure anddirect redirected radiation from the structure to a detection system,wherein:

the optical system is configured to apply a plurality of differentoffsets of an optical characteristic to radiation before and/or afterredirection by the structure, such that a corresponding plurality ofdifferent offsets are provided to redirected radiation derived from afirst point of a pupil plane field distribution relative to redirectedradiation derived from a second point of the pupil plane fielddistribution; and

the detection system is configured to detect a corresponding pluralityof radiation intensities resulting from interference between theredirected radiation derived from the first point of the pupil planefield distribution and the redirected radiation derived from the secondpoint of the pupil plane field distribution, wherein each radiationintensity corresponds to a different one of the plurality of differentoffsets.

2. The apparatus of clause 1, wherein the interference is such that acomponent of the detected radiation intensity containing informationabout the parameter of interest is enhanced relative to one or moreother components of the detected radiation intensity.3. The apparatus of clause 1 or clause 2, wherein the different offsetscomprise either or both of a different amplitude offset or a differentphase offset.4. The apparatus of any of clauses 1-3, wherein the different offsetscomprise at least one offset in a first sense and at least one offset ina second sense, opposite to the first sense.5. The apparatus of any preceding clause, wherein:

the different offsets are at least partially defined by apolarization-dependent optical element configured to modify an amplitudeor phase of radiation passing through the polarization-dependent opticalelement in dependence on the polarization of the radiation; and

the optical system is configured such that radiation from or forming thefirst point of the pupil plane field distribution passes through thepolarization-dependent optical element with a different polarizationthan radiation from or forming the second point of the pupil plane fielddistribution.

6. The apparatus of clause 5, wherein the polarization-dependent opticalelement comprises a retarder.7. The apparatus of clause 6, wherein the retarder is a variableretarder.8. The apparatus of any of clauses 5-7, wherein thepolarization-dependent optical element comprises a diattenuator.9. The apparatus of clause 8, wherein the diattenuator is a variablediattenuator.10. The apparatus of any of clauses 1-9, wherein the optical systemcomprises a polarizing beam splitter and the different offsets are atleast partially defined by different relative angles between thepolarizing beam splitter and either or both of a retarder and polarizer.11. The apparatus of any of clauses 1-10, wherein the different offsetsare at least partially defined by different splitting ratios of a beamsplitter.12. The apparatus of any of clauses 1-11, wherein the optical system isconfigured to cause the detection system to detect sets of radiationintensities resulting from interference between redirected radiationfrom a plurality of different pairs of first and second points in thepupil plane field distribution, each set of radiation intensitiescomprising a radiation intensity for each of the plurality of differentoffsets.13. The apparatus of clause 12, wherein each pair of points ispositioned mirror symmetrically with respect to each other about thesame line of mirror symmetry or point symmetrically with respect to eachother about the same symmetry point.14. The apparatus of any of clauses 1-13, wherein the optical system isconfigured to split a radiation beam into a plurality of radiation beamsand later recombine the plurality of radiation beams in order to causethe interference between the redirected radiation from the first andsecond points of the pupil plane field distribution.15. The apparatus of clause 14, wherein:

the splitting of the radiation beam into the plurality of radiationbeams creates multiple copies of a first pupil plane field distribution;and

the optical system forms a second pupil plane field distribution usingthe multiple copies of the first pupil field distribution.

16. The apparatus of clause 15, wherein the multiple copies of the firstpupil plane field distribution are rotated or flipped relative to eachother to form the second pupil plane field distribution.17. The apparatus of any of clauses 1-16, wherein the optical systemcomprises a beam splitter configured to split a radiation beam into afirst radiation beam and a second radiation beam, and the optical systemis configured such that the first radiation beam and the secondradiation beam propagate in opposite directions around a common opticalpath comprising a first branch and a second branch, the first radiationbeam propagating from the beam splitter to the substrate along the firstbranch and from the substrate back to the beam splitter along the secondbranch, and the second radiation beam propagating from the beam splitterto the substrate along the second branch and from the substrate back tothe beam splitter along the first branch.18. The apparatus of clause 17, wherein the optical system is configuredto perform at least one flip or rotation of the pupil plane fielddistribution of radiation propagating in the first branch or the secondbranch such that the image from the first radiation beam and the imagefrom the second radiation beam are respectively formed by radiationhaving pupil plane field distributions that are mirror symmetric orpoint symmetric with respect to each other.19. The apparatus of clause 17 or clause 18, configured so that theradiation beam input to the beam splitter comprises a pupil plane fielddistribution in which a first region of the pupil plane fielddistribution has been removed to leave only a second region of the pupilplane field distribution.20. The apparatus of clause 19, wherein the first region and the secondregion are oppositely oriented semicircles.21. The apparatus of any of clauses 1-20, wherein at least two of theplurality of radiation intensities corresponding to the plurality ofdifferent offsets are measured simultaneously in different measurementbranches.22. The apparatus of any of clauses 1-21, wherein at least two of theplurality of radiation intensities corresponding to the plurality ofdifferent offsets are measured at different times in the samemeasurement branch.23. The apparatus of any of clauses 1-22, wherein the parameter ofinterest comprises overlay.24. A lithographic system comprising:

a lithographic apparatus configured to perform a lithographic process;and

the metrology apparatus of any of clauses 1-23.

25. A method of measuring a structure formed on a substrate to determinea parameter of interest, the method comprising:

focusing radiation onto the structure and using a detection system todetect redirected radiation from the structure, wherein

a plurality of different offsets of an optical characteristic areapplied to radiation before and/or after redirection by the structure,such that a corresponding plurality of different offsets are provided toredirected radiation derived from a first point of a pupil plane fielddistribution relative to redirected radiation derived from a secondpoint of the pupil plane field distribution; and

the detection system detects a corresponding plurality of radiationintensities resulting from interference between the redirected radiationderived from the first point of the pupil plane field distribution andthe redirected radiation derived from the second point of the pupilplane field distribution, wherein each radiation intensity correspondsto a different one of the plurality of different offsets.

26. The method of clause 25, wherein at least two of the plurality ofradiation intensities corresponding to the plurality of differentoffsets are measured simultaneously.27. The method of clause 25 or clause 26, wherein at least two of theplurality of radiation intensities corresponding to the plurality ofdifferent offsets are measured at different times.28. The method of clause 27, wherein either or both of a variableretarder and a variable diattenuator are used to change an offsetapplied to the redirected radiation from the first point relative to theredirected radiation from the second point between the measurements madeat different times.29. The method of any of clauses 25-28, wherein the method is applied toa structure after a lithographic development step but prior to asubsequent etching step.30. The method of any of clauses 25-29, wherein the parameter ofinterest comprises an asymmetry in the structure.31. The method of any of clauses 25-30, wherein the parameter ofinterest comprises overlay.32. The method of any of clauses 25-31, wherein the detected radiationintensities result from zeroth order reflection from the structure.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens,” where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic, and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A metrology apparatus configured to measure a structure formed on asubstrate to determine a parameter of interest, the metrology apparatuscomprising: an optical system configured to focus radiation onto thestructure and direct redirected radiation from the structure to adetection system, wherein: the optical system is configured to apply aplurality of different offsets of an optical characteristic to radiationbefore and/or after redirection by the structure, such that acorresponding plurality of different offsets are provided to redirectedradiation derived from a first point of a pupil plane field distributionrelative to redirected radiation derived from a second point of thepupil plane field distribution; and the detection system is configuredto detect a corresponding plurality of radiation intensities resultingfrom interference between the redirected radiation derived from thefirst point of the pupil plane field distribution and the redirectedradiation derived from the second point of the pupil plane fielddistribution, wherein each radiation intensity corresponds to adifferent one of the plurality of different offsets.
 2. The apparatus ofclaim 1, wherein the interference is such that a component of thedetected radiation intensity containing information about the parameterof interest is enhanced relative to one or more other components of thedetected radiation intensity.
 3. The apparatus of claim 1, wherein thedifferent offsets comprise either or both of a different amplitudeoffset or a different phase offset.
 4. The apparatus of claim 1, whereinthe different offsets comprise at least one offset in a first sense andat least one offset in a second sense, opposite to the first sense. 5.The apparatus of claim 1, wherein: the different offsets are at leastpartially defined by a polarization-dependent optical element configuredto modify an amplitude or phase of radiation passing through thepolarization-dependent optical element in dependence on the polarizationof the radiation; and the optical system is configured such thatradiation from or forming the first point of the pupil plane fielddistribution passes through the polarization-dependent optical elementwith a different polarization than radiation from or forming the secondpoint of the pupil plane field distribution.
 6. The apparatus of claim1, wherein the optical system comprises a polarizing beam splitter andthe different offsets are at least partially defined by differentrelative angles between the polarizing beam splitter and either or bothof a retarder and polarizer.
 7. The apparatus of claim 1, wherein thedifferent offsets are at least partially defined by different splittingratios of a beam splitter.
 8. The apparatus of claim 1, wherein theoptical system is configured to cause the detection system to detectsets of radiation intensities resulting from interference betweenredirected radiation from a plurality of different pairs of first andsecond points in the pupil plane field distribution, each set ofradiation intensities comprising a radiation intensity for each of theplurality of different offsets.
 9. The apparatus of claim 1, wherein theoptical system is configured to split a radiation beam into a pluralityof radiation beams and later recombine the plurality of radiation beamsin order to cause the interference between the redirected radiation fromthe first and second points of the pupil plane field distribution. 10.The apparatus of claim 1, wherein the optical system comprises a beamsplitter configured to split a radiation beam into a first radiationbeam and a second radiation beam, and the optical system is configuredsuch that: the first radiation beam and the second radiation beampropagate in opposite directions around a common optical path comprisinga first branch and a second branch, the first radiation beam propagatingfrom the beam splitter to the substrate along the first branch and fromthe substrate back to the beam splitter along the second branch, and thesecond radiation beam propagating from the beam splitter to thesubstrate along the second branch and from the substrate back to thebeam splitter along the first branch.
 11. The apparatus of claim 1,wherein at least two of the plurality of radiation intensitiescorresponding to the plurality of different offsets are measuredsimultaneously in different measurement branches.
 12. The apparatus ofclaim 1, wherein at least two of the plurality of radiation intensitiescorresponding to the plurality of different offsets are measured atdifferent times in the same measurement branch.
 13. The apparatus ofclaim 1, wherein the parameter of interest comprises overlay.
 14. Alithographic system comprising: a lithographic apparatus configured toperform a lithographic process; and the metrology apparatus of claim 1.15. A method of measuring a structure formed on a substrate to determinea parameter of interest, the method comprising: focusing radiation ontothe structure and using a detection system to detect redirectedradiation from the structure, wherein: a plurality of different offsetsof an optical characteristic are applied to radiation before and/orafter redirection by the structure, such that a corresponding pluralityof different offsets are provided to redirected radiation derived from afirst point of a pupil plane field distribution relative to redirectedradiation derived from a second point of the pupil plane fielddistribution; and the detection system detects a corresponding pluralityof radiation intensities resulting from interference between theredirected radiation derived from the first point of the pupil planefield distribution and the redirected radiation derived from the secondpoint of the pupil plane field distribution, wherein each radiationintensity corresponds to a different one of the plurality of differentoffsets.
 16. The method of claim 15, wherein at least two of theplurality of radiation intensities corresponding to the plurality ofdifferent offsets are measured simultaneously.
 17. The method of claim15, wherein at least two of the plurality of radiation intensitiescorresponding to the plurality of different offsets are measured atdifferent times.
 18. The method of claim 15, wherein the parameter ofinterest comprises an asymmetry in the structure.
 19. The method ofclaim 15, wherein the parameter of interest comprises overlay.
 20. Themethod of claim 15, wherein the detected radiation intensities resultfrom zeroth order reflection from the structure.